Also on Monday, at least 13 people were killed after a suicide car bomb exploded at a police recruitment centre in the western Iraqi city of Ramadi, police said.

The suicide bomber struck outside the recruitment centre where a number of volunteers were seeking to join the police force, said Ahmed Ali, the local police captain.

At least 10 people were also reportedly injured in the attack.

The city is the capital of Anbar province, by far the deadliest for US troops.

The vast desert, with a largely Sunni population, forms a third of the country and is the heartland of its bloody insurgency.

US forces have been pulling some of its 30,000 troops out of the province to focus on stamping out violence in the capital, Baghdad, despite a US marine intelligence report leaked last week that said that Washington would need another division in Anbar to defeat insurgents there.

"We're very optimistic about the future of that province," said Major General William Caldwell, a US military spokesman.

"But [victory] will not occur in the next two to three months, it's going to be a much longer time period."